The invention relates to a composite membrane with a separation-active membrane layer and a supporting membrane. The invention relates further to a process for the production of a composite membrane of this type as well as to the use of a composite membrane of this type.
In the separation or retention of low-molecular substances from organic solutions, very energy-intensive preparation steps are usually used, for example, distillation, filtration, desliming, deacidification, deodorizing, bleaching, and so on. For example, in the production of edible oil a part of the edible oil is obtained by means of hexane extraction from the oil seed. In the extraction by means of this organic solvent (n-hexane) ca. 99% of the oil contained in the seed, but also other accompanying substances, are dissolved out. In a series of additional processing steps, solvents and accompanying materials must be separated after the extraction in order to obtain pure edible oil. Customarily this is done by means of a very complicated process in which, along with chemicals, large amounts of water are required. First, the solvent is removed from the extraction mixture, which comprises ca. 30% oil, at ca. 150° C. From the miscella arising in so doing, for example, phosphorus lipids are removed by precipitation with aqueous solutions of phosphoric acid and sodium hydroxide solution. After the oil has subsequently been washed, in order to remove remaining chemical residues, it is dried. Through the addition of activated carbon, dyes, such as chlorophyll and beta carotine, are removed. In the last treatment step, the material is deodorized with water vapor in a vacuum at ca. 200° C. in order to remove irritating odorous substances.
The production of an edible oil obtained by solvent extraction has previously been done via a conventional process which is very energy-intensive and in which great amounts of water and chemicals must be used. From AT 343 244 B, which corresponds to U.S. Pat. No. 4,062,882 A, a process for refining compositions of raw glyceride oil are known, said process using a porous membrane in order to separate components of different molecular weight. The separation limits specified in these steps lie between 1,500 and 200,000 in regard to the molecular weight. Preferred is a range from 10,000 to 50,000 mw (mw meaning molecular weight). The membranes are polycyinitrile [sic] membranes and serve for separating phospholipids which aggregate in relatively large amounts in the solution to be separated.
In DE 31 51 966 C2 which corresponds to U.S. Pat. No. 4,414,157 A, the use of porous polyimide hollow fiber membranes for the separation of phospholipids from n-hexane is described. The membranes used have a separation limit between 10,000 to 100,000 mw.
In the Journal of Membrane Science, 282 (2004), pages 103 to 116 a composite membrane for the separation of oil/n-hexane mixtures is described. The composite membrane consists of a polyacrylnitrile supporting membrane and a separation-active layer of polydimethylsiloxane (PDMS). For this, a high-molecular prepolymer with vinyl groups is used. The cross-linking is done by hydrosylilation with the aid of a platinum catalyst and a cross-linking agent which consists of short-chain PDMS with hydride groups. The stability of this membrane in edible oil/n-hexane mixtures is however relatively brief.
In the state of the art no membranes have been specified previously which have a high separation power for low-molecular substances with a separation limit of under 2,000 (preferably between 200 to 1,500) in oil/n-hexane mixtures with simultaneous long-term stability of the membrane. Furthermore, the preparation processes of edible oil/n-hexane mixtures are very energy-inefficient and must furthermore be carried out at higher temperatures so that the quality of the oil can be reduced. Furthermore, conventional processes require the use of chemicals such as, for example, sodium hydroxide solution which subsequently must be removed from the oil once again in a complicated manner. Thus, the consumption of water and energy is relatively high for the conventional processes.